Stretching of polyethylene terephthalate film



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lSTRETCl-IING OF POLYETHYLENE TEREPHIHALATEAFILM Filed- May l2. 1952 16 Sheets-Sheet 10 FIG I2 cRYsTALLnzATloN AT 972 cM" INVEIVTORS ALBERT HERSHBERGER ARTHUR C. SCARLETT ATTORNEY.

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. INVENTORS ALBERT HERSHBERGER BY ARTHUR C. SCRLETT A TTORNEY.

Feb. 18, 1958 A. c. scARLl-:TT 2,823,421

l STRETCHING 0F POLYETHYLENE TEREPHTHALATE FILM Filed May 12. 1952 16 sheets-Sheet 14 v l FIGA@ cRYsTA-LLIZATION ssc a i I av I f AFTER M.D. sTRETcl-lln c loo 75 80 85 90 95 |00 l |05 HO 'r M P Tu INVENTORS E ER A R E` C ALBERT HERSHBERGER 8 ARTHUR C. SCARLETT @My/@J4 A TTORNE Y.

HALF-TIME, SECOND'S Feb. 18, 1958 A. c. scARLE-r'r 2,823,421

sTRETcHING oF 'POLYETHYLENE TEREPQTHALATE FILM Filed may 12. 1952l 16 sheets-,sheet 15 U Flai? CRYSTALLI {.ATION AFTER'.M.D.STFET(IHIDGZAZZX 'Y I leo v f V -|40 l |20 l looI v 60 I Y E TEMPERATURE c. Y INVENToRs ALBERT HERSHBERGER ARTHUR c. scARLETT A TTORNE Y.

Feb. 18, 1958 A. c. scARLETT 2,823,421

sTRETcl-IING oF POLYETHYLENE TEREPHTHALATE FILM 16 Sheets-Sheet 16 Filed May l2. 1952 O R H S RRE/. OER TB HA m c 5 SS OVR INEC. 1H U X R a Mm O LM G m A N N 8 O l I H C. T N o 5 E Z E 9 R 8 L T T S A D. R A E M w P G T M I S R E .r v w R F C A Q 9 a O O O O O O O n O O O O O O O O O O O O 8 6 4 2 O 8K 6 2 O 8 6 4 2 O 8 6 4 2 4 3 3 3 3 3 2 2 2 2 l l l I l ATTORNEY.

f e 2,823,421 ice Patented Feb. 1s, 195s STRETCHING F POLYETHYLENE TEREPHTHALATE FILM Arthur C. ScarlettBuffalo, N. assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application May 12, 1952, Serial No. 287,354

6 Claims. (Cl. 18-57) This invention relates to a process of improving the physical properties of a polyester film, and, more particularly, to a process of improving the physical properties of polyethylene terephthalate film by stretching.

Polyethylene terephthalate, described and claimed in U. S. P. 2,465,319 to Whinfield and Dickson, may be prepared by the condensation of ethylene glycol and terephthalic acid, or, preferably, by carrying out an ester interchange reaction between ethylene glycol and a dialkyl ester of terephthalic acid, e. g., dimethyl terephthalate. Films of polyethylene terephthalate may be prepared by extruding the molten polymer through a narrow orifice land chilling the polymer in film form. The opening of the orifice is adjusted in ac-cordance with the caliper desired. Such film inherently has a number of excellent physical properties which make it useful in a great variety of applications, e. g., packaging, electrical applications as a dielectric, protective coverings, glass replacement, etc. However, certain physical properties, such as tensile strength, impact strength, iiex life, water vapor, and organic vapor permeability and tensile modulus a-re not competitive with those of other types of film compositions.

- This prohibits the wide use of unoriented polyethylene terephthalate film in many applications, especially in the electrical industry.

An object of the present invention, therefore, is to produce -a polyethylene terephthalate film having an outstanding combination of physical, chemical and electrical properties. A further object is to produce a polyethylene terephthalate film having substantially equivalent mechanic-al properties as measured in both the machine and transverse directions. A further object is to provide Va process of biax-ially stretching polyethylene terephthalate` film. A still further object is to provide a process of fbiaxially stretching polyethylene tercphthalate film continuously at appreciable stretching rates. Other objects will be apparent from a fur-ther description of the invention hereinafter.

The above objects are accomplishedaccording to the present invention by extruding molten polyethylene terephthalate to form an amorphous film and thereafter continuously longitudinally stretching the film at least at the rate of 400% per minute no greater lthan 3.25, times (3.25)() at a temperature between S090 C., preheating the longitudinally stretched film at a temperature between 9095 C., continuously transversely stretching the film substantially the same amount at at least 400% per minute at a temperature within the range 95 1l0 C. to produce a balanced film, and 4thereafter heat-setting the biaxially stretched film at a temperature within the range 150-250 C. It is to be understood that the above temperatures represent those to which the film is subjected, i.e., the environment about the film. Owing to the heat of stretching generated'within the film, the actual temperature of the film at any time during the process is usually higher than the temperature of its irnrnediate environment.

`Conventional film extrusion apparatus and conventional 2 apparatus designed to stretch continuous web material longitudinally and transversely may be used to carry out the process `of the invention hereinabove outlined. A convenient arrangement ofapparatus particularly adapted to -the practice of the present process will now bedescribed withreference to Figuresr 1 and 2 of the accompanying drawings wherein areillustrated diagrammaticaliy the arrangement of fihn ,casting and longitudinal stretching apparatus (Fig. l), and vthe transverse stretchingv and heat-setting apparatus (-Fig. 2). The remaining figures of the drawing, viz., Figs. 3 toY l8-inc1usive,.are` graphs illustr-ative of critical conditions which characterize the process of this invention. e

Referring to Figures 1 and 2, molten `polyethylene terephthalate is extruded vat a temperature of 270-315 C. through a narrow slot orifice of hopper V vertically downward onto a -cool drum W maintained atf60,"80 C. The linear speed of the surface yof the drum is in the neighborhood of 1.5-10 times faster than the linear rate of extrusion of the film. After setting, the film P, which is about 20 wide, is continuously stretched longitudinally and then transversely in an apparatus essentially vcomposed of two main parts, namely, a longitudinalV stretching section and a transverse stretching section. The longitudinal stretching section is composed of 19 horizontal rolls in parallel arrangement and lying in different vertical and horizontal planes. The rst five rolls., A-E inclusive, are positively driven slow rolls; the next 9 rolls, l-9 inclusive, are closely spaced idler rolls andnoty positively driven; and the last 5 rolls, F-J inclusive, are I ositivelyV driven fast rolls. Actual stretching is carriedout-over the idler rolls, andthe extent of longitudinal stretch is determined by the difference in linear speed of the posi-A tively drivenslow rolls andfast rolls. t All of the rolls are internally heated (by any conventional expedient, not shown) and maintained within the range 90 C., and usually within the range 90 C. In the specific apparatus employed to illustrate the process of the present invention, the length of film actually in the stretching rolls at all times is about 21'; This lengthV is distributed .over the slow rolls, idler rolls and fast rolls as follows: 6', 9' and 6, respectively.

The transverse section of the stretching apparatus. is essentially dividedv into four zones. The entire section is composed of a tenter frame having a chain of tenter clips on both'sides ofthe film. As the filmemerges from the longitudinal stretching section, it is directed between parallel rows 10 and 11 o f tenter clips; and the tenter clips grasp the edges of the longitudinally stretched `film and move outward to stretch the film transversely. TheV first zone yis represented by the distance from the end of maintain substantially the-temperature of the film as it emerges from the longitudinal stretchingsection, it will e be hereinafter referred to as the preheatingzone.

This zone is about l0 in length andthe temperature is Within the range '95 C. The second zone represents the section between the beginning and end of transverse stretching. This zone is 20 in length, yand the tem,-

heat-setting Zone in which the film, as it passes through:

the-housing 12, is subjected to an elevated temperature within the range -225 C., while the film is maintained under transverse tension. The heat-setting zone isY 20 in length. The final zone of the transverse stretching l section is v open to the atmosphere andserves to cool the film gradually. This zone is 10': in length;

The molten polyethylenev terephthalate must be casty under'conditions'such'that the formed film, whenset, is'v substantiallyiainorphous"(non=crystalline). YThis is mostI conveniently accomplished by extruding the melt, as just described, onto a casting drum maintained at a temperature suiciently low to effect rapid quenching or chilling of the polymer from the molten state. The film remains substantially amorphous up to the point of longitudinal stretching. Thereafter, the rate of crystallization increases as the film is subjected to stretching at elevated temperatures, and it is the rate of crystallization in combination with the degree of orientation which iniiuences the conditions of the biaXial stretching process of the present invention. Generally, as a film becomes more crystalline, i. e., increases in density, the work required to stretch the lm greatly increases in contrast to the work required to stretch a substantially amorphous lm.

In continuously biaXially stretching polyethylene terephthalate film in an apparatus of the type described, the film is preferably stretched at rates at least 400% per minute and, generally, within the range from 1,000- l,500% per minute. Obviously, in a continuous process, the highest rates are desirable in order to obtain a high rate of film production and stretching rates as high as 2,000-4,000% per minute may be used in the present process. Hence, the process of the present invention delines the critical conditions for obtaining biaXially oriented polyethylene terephthalate film having the optimum combination of physical, chemical and electrical properties at high stretching rates. In the stretching apparatus described hereinbefore, a stretching rate of 400% per minute is equivalent to about yards per minute, whereas a stretching rate of 2,000% per minute corresponds to about 100 yards per minute in the specific apparatus described herein.

The critical temperature limitations of the biaxial stretching process of the present invention are governed mainly by three points of consideration, namely: (l) eiciency of orientation, (2) work required to stretch, and (3) rate of crystallization of the polyester film. As a general requirement for continuous stretching of polyethylene terephthalate film at appreciable rates, i. c., at least 400% per minute, longitudinal stretching 1 must be carried out at a temperature of at least 80 C.; and the transverse stretching step must be carried out at a somewhat higher temperature'than that of the first direction step Yin order to maintain the Work of stretch at a minimum and avoid film breakage. This minimum temperature is established by observing the temperature at which a discontinuity occurs in the curve of a first derivative thermodynamic quantity of the polymer with temperature. This may be observed from a plot of density, linear eX- pansion, specific volume, specific heat, sonic modulus, initial modulus or index of refraction `against temperature. Figure 3 contains a plot of initial modulus vs. temperature in which is shown the change in the direction of the curve occurring at a temperature in the neighborhood of 85 C. Generally, depending upon the intrinsic viscosity of the polymer, the minimum temperature above which the first i direction stretching is carried out is within the range 80 85 C. Hence, as will be illustrated hereinafter, the first direction or longitudinal stretch must be carried out at a temperature within the range 80-90 C. so that the Work required to stretch the film is at a substantial minimum; and at this temperature, the film draws homogeneously over the entire area of the film under tension. Furthermore, the second direction or transverse stretch must be carried out at a temperature somewhat greater than that at which the film is stretched longitudinally. This is because the film is oriented in the machine direction, which produces a stronger film; and with crystalliza- 1The term longitudinal stretching will be used herein to mean the direction in which the film is stretched rst, and transverse stretching Wlll be referred to as the second direction stretch. Obviously, the film could be stretched first in the transverse. direction and then in the `longitudinal` or machine direction; but when employing rolls to stretch the film longitudinally, the rolls would have to be excessively long if the lm were first stretched transversely.

tion taking place, the amount of work required for stretching in the second direction is increased. Hence, by carrying out the second direction stretch at higher temperature, i. e., 5-20 C. higher, the work required for stretching is maintained at a minimum.

For the purpose of further considering the behavior of polyethylene terephthalate film upon stretching, reference is had to Figure 4 which represents a stress-strain diagram for amorphous polyethylene terephthalate film, this diagram embodying a number of definitions which willbereterred to hereinafter. The plot of Figure 4 begins with a steep straight line where the stress is proportional to the strain. This ratio at low elongations is termedV the initial modulus M and is a measure of film stiiiness. The sudden change in direction of the curve is called the yield point, YP, which is located by reference to the tension and elongation at that point. Frequently, asecond smaller peak occurs which is termed the sec- `ogndraryyield point, YPZ. Beyond this point, the film elongates with little 'or no increase in tension. The lowest tension level of the region is called the stretchingv force, SF.y rThe point at which the film begins to offer 'resistfance to stretching is called reinforcing point, RP. Finally, vat the end of the curve is the tensile strength T, [and breaking elongation, E. The area under the curs/ e is representative of the Work of stretching, WS. ln' order' to ascertain the work required to stretch amorphous polyethylene terephthalate film at various temperatures below 80 C., a series of stress-strain diagrams were plotted to obtain these data. Figure 5 is ay .seriesof stress-strain diagrams at temperatures ranging from 25 to 90 C. Table l is a tabulation of the Vdata, employed to plot the diagrams of Figure 5, includingthe'various data obtained from the plots.

TABLE 1 i Effect of temperature on stress-strain properties of amorphous Lmstretched polyethylene terephthalate Yield, Stretch Work to Temp., C. Modulus, Point Force, Stretch 3 X p. s. i. Tension, p. s. i. (in lbs./in.3)

As shown in the stress-strain diagrams of Figure 5, amorphous polyethylene terephthalate film does not draw homogeneously at temperatures below 80-85 C. This means that when tension is applied longitudinally, drawing takes place from a transverse line and is not effected uniformly over the entire surface of the film. This is indicated in the stress-strain diagrams at 25 C., 40 C. and 60 C., wherein there is the appearance of a secondary yield. point, the tension increasing almost linearly to a high load value, suddenly decreasing about 25% and then increasing sharply a second time and finally decreasing to some constant level at which it elongates with little or no change in load. This secondary yield point is believed to be due to the formation of a second line of drawing. Furthermore, Table l clearly shows the minimum amount of work required to stretch amorphous polyethylene terephthalate film at temperatures within the range 90 C.

In contrast with the work required to stretch a crystallized unstretched polyethylene terephthalate film, a sample of an amorphous film was exposed to a temperature of for one hour to effect crystallization. Figure 6 TABLE 2 Eect of temperature on stress-strain properties of crystallized Lmstretched vpolyethylene terephthalate Modulus, Yield Stretch Work to Temp., C. p. s. Point, Force, Stretch 3X p. 's. l. p. `s; i. (i111bs./in.3)

300, 000 9, 225, 000 6, 80 195,000 Y 168, 000 6, 00 105, ooo 4, 90o 61, 000 3, 000 36, 000 2, 20 16, 000 1, 00

In the `first direction (longitudinal) stretching step of the process-of the present invention, the foregoing discussion has lbeen devoted to considering the effect of temperature ou lthe work required for stretching the polyester film, 80 90 C. being optimum for stretching amorphous film. The efiicie'ncy of orientation at various stretching ktemperatures is also a critical `factor with respect'to producing `a biaxially stretched film'having the optimum combination ofphysical, chemical andelectrical properties. Various techniques may be employed to measure orientation of the polyester film such as X-ray diffraction, polarized infrared absorption, swelling in water,`heat shrinkage, comparison of MD (machine direc'tion) and TD (transverse direction) physical properties su'ch as tenacity or elongation, andmeasurement of birefringence. For measuring orientation of polyethylene terephthalate film, the measure of birefringence was selected. `Birefringence is a dimensionless number and is a direct measure of the Vdifference of the refractive indices of the film parallel to and perpendicular to the axis of orientation. When a birefringent film such as oriented polyethylene terephthalate transmits a beam of plane polarized light which strikes perpendicular to the axisY of orientation (normal to the plane of the sheet, for example), the line is split into two beams polarized at right angles to each other, one of which travels faster than the other. is ahead of the other when they emerge from the film is known as lthe retardation of the sample (usually expressed-in millirnicrons) and is related to the film thickness andto birefringence, An, by the equation,

' Retardation: thickness birefringence the greater the degree of orientation. Figure 8 is a plot o'f birefringence vs. draw temperatureHfor a sample of polyethylene terephthalate film (0.002 in thickness) which has been drawn 3.5)( in one direction. The film sampleswere 6" by 10"; and after drawing at the various-temperatures indicated, the birefringence measurements-were;made upon center portions-of the samples.

Because the film could not be drawn successfully ati-'the The distance that one of these beams the orientation of samples .of'

`6 lower temperatures, the samples which were lto Abe stretched at 25 and 50 C. were tirst heated for 'l5 seconds at'120" C. and then air quenched to increase Itheir drawability.

As shown in Figure 8, there is a rapid drop in efficiency of orientation as the temperature increases appreciably beyond -85 C. Hence, in conjunction with the foregoing considerations with respect to Athe amount of work required-to stretch the amorphous polyester film, a temperature within the range 80-90 `vC. is optimum for the first direction or longitudinal stretch. Preferably, the temperature is about- C.

In general, with respect to thetemperature range within which polyethylene terephthalate film is stretched in the first or longitudinal direction, stretching at temperatures above C. results in substantially vno orientation. On the other hand, stretching at temperatures below 80 C. results in substantially non-uniform orientation in view of the fact that drawing takes place from various lines of demarcation; and stretching is not uniform over the 4entire cross-sectional area of the film.

With respect to stretching one-way stretched polyethylene terephthalate film in the transverse direction, the main factors to be taken into consideration are: (1) the work required to stretch the film in the'second direction, and (2) the rate at which the film is crystallizing. Actually, these factors are directly connected because the more crystalline thefilm is, the greater is the workrequired lto stretch in the second direction. Furthermore, the degree of orientation which has been imparted to the film after the first direction stretch also increases the amount of work required to stretch'the 'film in the-second direction. Hence, raising the temperature` of the film during the `second direction or transverse'stretch serves to maintain the Workrequired ata minimum in addition to reducing film breakage to a negligible degree.

VGeneral measurements of the density of a'film stretched 3 inthe longitudinal direction have'indicated that'the film is about l0-l4% crystalline. Furthermore,varter a polyethylene terephthalate film has been stretched 3X in bothdirections, vitis about 20-25% crystalline; 'and the heat-setting step in the neighborhood of 200 C.. produces a -finalfilmwhich is about 4Q-42% crystalline.

As polyethylene terephthalate filmis eXposedto increasing temperatures, crystallization is initiatedyand the'rate of crystallization 4increases as the temperature increases up to a temperature in the neighborhood of 200 C. As thetemperature increases appreciablyabove 200 C., .the rate of crystallization again decreases as at lower'lte'mperatures. Furthermore, vthe combination ofexposureito elevated temperatures and stretching in lone or both directions further increases the rate of crystallization .of the polyester film.

'In thefpast, density changes' have been used to measure the crystallinity of various polymersystems. 4For-'example, amorphous unstretched polyethylene terephthalate filmhas a density at 30 C. of 1.331 :gms/cc. .X-ray studies give a density calculatedfrom the dimensionsof the trifclinic unitcell as 1.47 gms./ cc. for the' theoretically pure-crystalline polymer. Polymer having adensity'between v.1.331 and 1.47 exhibits varying degrees ofcrystallinity. Forthe purpose of measuringrapid' changes in the crystallinity of .polyethylene terephthalate, th'emeasurement ofdensity is inadequate. fIn order to vfollow -the'rapid changes in .the crystallinity of unstretched and Vstretched polyethylene'terephthalate film'upon exposure toelevated' temperatures, infrared absorption measurements have vbeen employed. The infrared absorption was correlated quantitatively with the density `of polyethyleneterephthalate for varyingv degrees of crystallinity, and qualitatively with thechange in X-ray'difraction pattern with crystallization of the polyester. Samples vof polyethylene'tere'phthalate films 0.002 and 0.00.1.respectively,v in thickness were employed. .Infrared spectraland Xray .diira'cti'on patterns wereobtained onlthe .film samples-beforehand' '7 after heat treatment for 1% of an hour at 195 C. Figures 9 and 10 show the comparative results. The frequency of light waves in terms of the number of such waves occurring in one centimeter length is called the wave number and is measured in reciprocal centimeters abbreviated cml. Figure 9 illustrates the Wide ditference in percent transmission of infrared at the frequencies, 1340 cm."1 and 972 cmi-1. For the purpose of following the course of crystallization, the latter frequency was chosen for use because it presented less possibility of interference by variation in the absorption due to water vapor in the atmosphere. Figure 10 shows an X-ray difraction pattern of the amorphous film and the crystalline film (after heat treatment for T of an hour at 195 C.).

Since density changes have been used for measuring crystallinity, the change in infrared absorption at a frequency of 972 cm.1 was correlated with change in density. In Figure 11, the optical density at 972 cm.-1 divided by that at 795 cm.-1 is plotted for films of varying density. The optical density at 795 cm.1 was not appreciably influenced by crystallization, and division by it served to allow elimination of the thickness variable with the samples used. The infrared measurements were made at 25 C. The varying densities were obtained by progressive heat treatments. The densities were measured in a density gradient tube using heptane and carbon tetrachloride at 30 C. and are known to $0.001 gram per cc. The straight line of Figure 11 shows proportionality between optical density and density for the crystallization process.

The apparatus employed for obtaining isothermal crystallization time curves was a standard Perkin-Elmer model l2-C infrared spectrometer with sodium chloride optics. In the center was an insulated duct for air leading from heaters below to the insulated sample compartment between the instrument housings. A blower provided forced circulation in the system. A thermocouple i11- serted in the air stream in the sample compartment leads to a recorder-controller. The controller served to keep the temperature in the sample compartment constant to 1 C. by control of an appropriate amount of the electrical heating available from the air heating chamber located below. Suitable baflies of copper sheet prevented overheating of the spectrometer proper. A small housing on top of the sample compartment served as an air lock for very rapid introduction of the sample into the light beam of the spectrometer by gravity fall. A sample holder of low heat capacity may be seen placed against the instrument casting at the extreme left. The sample holder is of a size sufficient to make the temperature of the part of the thin film sample in the light beam independent of that of the holder metal itself. In use, the sample holder was placed in the air lock chamber; a sliding door allowed the holder to fall suddenly into supporting ways within the main sample compartment. The ways are situated so that free air circulation is provided around the sample and the latter is reproducibly positioned in the light beam. Sodium chloride windows allow the light to pass through the sample compartment.

To obtain crystallization time curves, the apparatus was rst brought to temperature equilibrium at the temperature chosen for measurement. A crystallized sample was introduced into the beam, and the monochrometer was carefully adjusted to the frequency of the minimum of the absorption band at 972 cm.1. The slit-width used was 0.200 mm., giving a resolution of about 4 cm.1 and a signal to noise ratio such as to allow determination of optical density to approximately 0.005 unit. The trial sample was removed, and the recording system of the spectrometer adjusted for zero and infinite optical density on the logarithmic chart paper. The time travel or axis of the chart was started. The sample for measurement was chosen to give optical densities ranging from about 0.3 to 0.5; about 0.001 inch was the proper thickness. It was placed in the holder and introduced into the air lock. The sliding door was operated; the sample dropped suddenly in free fall into the holding ways below. The passage of the holder through the beam served to mark zero time on the chart record. The thin film is estimated to reach the temperature of measurement in less than a second. The amorphous optical density recorded initially after fall to the rest position increased as crystallization progressed, finally tapering off to a limiting value characteristic of the crystallized sample. Figure 12 is a typical experimental record obtained in this manner at 140 C.

For crystallization measurements, all samples were taken from the same lot of polymer in the form of a cast film of 0.001" thickness. The intrinsic viscosity was about 0.61, and the polymer was cast from a melt at about 300 C. and quenched to room temperature. No evidence of the presence of crystallinity could be obtained in this material by X-ray examination or by measurement of the density even one year after it was Cast.

Several samples of film were crystallized and the crystallization curve of each recorded at temperatures from C. to 240 C. The continuously determined data for one curve at each temperature are plotted in Figure 13. In this figure, the curves start at different optical densities because the absorption is taking place at different temperatures and because the samples varied in thickness initially. Table 3 summarizes the half-times for crystallization to the observed limiting values at various temperatures and includes several measurements at each temperature. The limiting values of density of polyethylene terephthalate at various temperatures were obtained by maintaining the lm at the given temperature for a period equivalent to 10 half-times.

TABLE 3 Crystallzation of polyethylene terephthalate Half-Time Temperature, C.

Mn. Sec.

The density of unstretched polyethylene terephthalate film after 10 half-times at various temperatures is recorded in Figure 14. 

5. A PROCESS FOR PREPARING POLYETHYLENE TEREPHTALATE FILM WHICH COMPRIESE CONTINUOUSLY CASTING A SUBSTANTIALLY AMORPHOUS POLYETHYLENE TEREPHTALATE FILM, CONTINUOUS LONGITUDIALLY STREACHING SAID AMORPHOUS FILM AT THE RATE OF AT LEAST 400% PER MINUTE NO GREATER THAN 3.25X AT A TEMPERATURE WITHIN THE RANGE OF 80*-90*C. THEREAFTER COMTINUOUSLY TRANVERSLY STRETCHING SAID FILM TO SUBSTANTIALLY THE SAME EXTENT AT A RATE OF AT LEAST 400% PER MINUTE AT A TEMPERATURE WITHIN THE RANGE OF 95*-110* C., AND CONTINUOUSLY HEAT-SETTING THE RESULTING BIAXIALLY STRETCHED FILM AT A TEMPERATURE WITHIN THE RANGE OF 150*C250*C. WHILE MAINTAINING SAID FILM UNDER TRANSVERSE TENSION. 